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13 more things: Prehistoric hothouse

THE Eocene ran from 56 million to 34 million years ago. Geological evidence from the early and middle part of this period offers troubling news&colon; the average temperature in the tropics at this time could have been as high as 40°C while the poles were at temperatures of 15 or 20°C. None of our climate models accounts for how this “Eocene hothouse” might have arisen (New Scientist, 21 June 2008, p 34).

Whichever way you look at the Eocene enigma, it is bad news for life on Earth. For a start, any tweaks we make to our climate models to account for it will produce scarier predictions of warming. Secondly, it suggests that there is no feedback mechanism that will stabilise a warming world against runaway climate change. And third, there is geological evidence for plant extinctions in the Eocene.

If the modern Earth goes the same way and plants in the tropics start dying, that will provide yet another way for atmospheric carbon dioxide levels to rise faster. The Eocene hothouse anomaly suggests that our worst-case scenario is probably optimistic to say the least.